Plasmonic quantum well laser

ABSTRACT

A plasmonic quantum well laser may be provided. The plasmonic quantum well laser includes a plasmonic waveguide and a p-n junction structure extends orthogonally to a direction of plasmon propagation along the plasmonic waveguide. Thereby, the p-n junction is positioned atop a dielectric material having a lower refractive index than material building the p-n junction, and the quantum well laser is electrically actuated. A method for building the plasmonic quantum well laser is also provided.

FIELD

The present disclosure relates generally to a plasmonic quantum welllaser, and more specifically, to a plasmonic quantum well laserincluding a plasmonic waveguide. The disclosure relates further to amethod for building a plasmonic quantum well laser.

BACKGROUND

Today's computing devices integration level is getting higher and higherwith every generation of processing devices. The internal communicationspeed of such devices is pushing the physical limits to a maximum. As analternative for an electrical signal-based communication, light ofphoton-based chip internal communication infrastructures areenvisioned—and in some cases already practically used—to increase thebandwidth of chip-internal communication systems. Light as acommunication medium may be used within one single semiconductor devicelayer or—in 3D chip designs—also as a means of communication from onelayer to another layer of the same integrated circuit. In particular,for a communication within one single semiconductor device layer,low-loss surface plasmons might be foreseen as an alternative tophotonic modes for short range (<100 um) communications as propertiessuch as speed and confinement might outweigh the high optical absorptionassociated with the plasmonic waveguide. In such a scheme, plasmoniclasers may be instrumental as a candidate for a radiation-source inlarge-scale integrated (LSI) circuits using alternatives to pureelectric communications aids.

Very recently, ultra-small lasers based on collective charge ofselectors at the interface between the metal and a semiconductor, calledsurface plasmon polaritons (SPPs), have been proposed and experimentallydemonstrated. However, the strong mode confinement of SPP's modes at thesemiconductor-metal interface strongly reduces the overlap of activegain material with a propagating plasmon, while the presence of metaladds to the optical losses and a large fraction of bulk semiconductormaterial typically do not contribute to stimulated emission. Theconsequent increase of the lasing threshold together with inferencingplasmon propagation losses has rendered plasmon lasers inefficientcompared to their photonic counterparts.

Photonic lasers, based on semiconductor quantum well structures, havealready been proposed in the 1970s and have become one of the mostimportant semiconductor laser technologies today. Although, the overlapof the optical modes with the quantum well gain material is reducedcompared to bulk semiconductors, the high gain and temperature stabilityprovided by quantum well gain material typically over-compensate theseintrinsic current losses. A main limitation for achieving integrationdensities of photonic components compared to micro-electronics is themuch larger size of photonic devices. A conventional semiconductor laseris typical of dimensions in the order of 100s or micrometers, which isabout 10,000× greater than a typical electronic MOSFET switch in anadvanced computing node. The ability to scale photonic components islimited by the diffraction of light, by dimension less than thewavelength of light in a given material; the optical mode will leak out.

Thus, there may be a need for VLSI chips to have a non-electriccommunication mechanism that requires less space than typicallight-based photonic concepts.

SUMMARY

According to one aspect of the present invention, a plasmonic quantumwell laser may be provided. The plasmonic quantum well laser may includea plasmonic waveguide. The plasmonic quantum well laser may comprise ap-n junction structure extending orthogonally to a direction of plasmonpropagation along the plasmonic waveguide. The p-n junction may bepositioned atop a dielectric material having a lower refractive indexthan the material building the p-n junction, and the quantum well lasermay be electrically actuated.

According to another aspect of the present invention, a method forbuilding a plasmonic quantum well laser may be provided. The methodcomprises providing a SiO₂ layer over a Si bulk material. A Si layer isdeposited atop of the SiO₂ layer. The method may also comprisepatterning an Si structure allotted for a gain region into the Si layer,such that the Si structure forms a pattern atop the SiO₂ layer, coveringthe Si structure with an SiO₂ template layer, and building an opening inthe SiO₂ template layer from one side and etching away the Si pattern,thereby building a cavity within the SiO₂ template layer.

Furthermore, the method may comprise epitaxially growing horizontally afirst semiconductor portion of a p-n junction extending from a siliconseed exposed within the SiO₂ template towards the opening until apredefined horizontal extension is reached, growing horizontally asecond portion of the p-n junction through the opening extending from anend of the predefined horizontal extension facing the opening until thecavity is filled, such that the p-n junction builds the gain region ofthe plasmonic quantum well laser, and depositing a dielectric materialover the p-n junction. Last but not least, the method may comprisedepositing a plasmonic waveguide over the dielectric material along alongitudinal extension of the p-n junction.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

It should be noted that embodiments of the invention are described withreference to different subject-matters. In particular, some embodimentsare described with reference to method type claims, whereas otherembodiments are described with reference to apparatus type claims.However, a person skilled in the art will gather from the above and thefollowing description that, unless otherwise notified, in addition toany combination of features belonging to one type of subject-matter,also any combination between features relating to differentsubject-matters, in particular, between features of the method typeclaims, and features of the apparatus type claims, is considered as tobe disclosed within this document.

The aspects defined above, and further aspects of the present invention,are apparent from the examples of embodiments to be describedhereinafter and are explained with reference to the examples ofembodiments, but to which the invention is not limited.

Embodiments of the invention will be described, by way of example only,and with reference to the following drawings:

FIG. 1 shows a block diagram of an embodiment of the inventive plasmonicquantum well laser.

FIG. 2 shows a general schematic illustration of a plasmonic waveguideon top of a horizontally oriented quantum well in an embodiment.

FIG. 3 shows a dielectric coated metallic nano-wire waveguide on asimple p-n junction in an embodiment.

FIG. 4 shows an alternative embodiment, in which the metallic nano-wireis only positioned on one side of the p-n junction.

FIG. 5 shows again an alternative embodiment with a metallic nano-wirecoated with dielectric material, and positioned on top of a p-n regionwith multiple quantum wells.

FIG. 6 shows a further alternative embodiment in which a metallicnano-wire waveguide is shown on a layer of very thin dielectricmaterial, positioned on top of a pre-patterned gain material.

FIG. 7 shows an additional alternative embodiment, wherein the metallicwaveguide is positioned on a pnp double junction.

FIG. 8 shows a further implementation option, in which the metallicwaveguide is positioned atop a pnp double junction with a plurality ofquantum wells in an embodiment.

FIG. 9 shows another embodiment with a plane metallic layer over aburied p-n junction.

FIG. 10 shows a special embodiment of the quantum well laser with themetallic slot waveguide.

FIG. 11 shows process steps of an embodiment of a method for buildingthe plasmonic quantum well laser using template assisted selectiveepitaxy.

DETAILED DESCRIPTION

In the context of this description, the following conventions, termsand/or expressions may be used:

The term ‘plasmonic quantum well laser’ may denote a semiconductor diodeor similar in which the active region of the device is so narrow thatquantum confinement occurs. The diodes may be formed in compoundsemiconductor materials that (quite unlike silicon) are able to emitlight and, in particular, plasmons efficiently. The wavelength of thelight emitted by a quantum well laser may be determined by the width ofthe active region rather than just the bandgap of the material fromwhich it is constructed. This means that different wavelengths mayrather be obtained from quantum well lasers than from conventional laserdiodes using a particular semiconductor material. The efficiency of aquantum well laser is also greater than a conventional laser diode dueto the stepwise form of its density of states function. However, herethe focus is on excited plasmons that may be confined to much smallerregions than related photons. In contrast to typical photon-based lasers(Light Amplification by Stimulated Emission of Radiation), plasmonicquantum well lasers function with plasmons instead of photons. Thenotion “laser” is also used in this context to the same stimulatedemission of radiation—not of photons, but of the quasi-particlesplasmons. In this sense, also the term SPASER (semiconductor plasmonamplification of stimulated emission of radiation) may be used as asynonym for ‘plasmonic quantum well laser’ in the case where the modewould be of plasmonic nature.

For plasmons, it is not the wavelength that gets shorter if compared torelated photons. The plasmon may have the same frequency/wavelength asthe related light-photon, i.e., they can couple. However, the plasmonmay be confined to a much smaller volume and is, therefore, not subjectto the diffraction limit of light. The wavelength, or better theresonance frequency, may still be the same and match the gain materialwavelength/frequency and the cavity resonance wavelength/frequency, justlike a “normal” laser.

The main difference is that there is a stimulated emission of plasmonsand not a stimulated emission of photons. One may say that a maser(microwave amplification of stimulated emission of radiation) relates toa laser (photon-based) in the same way as a laser relates to a plasmonicquantum well laser or plasmonic laser.

In the here proposed concept, the term ‘quantum well’ might be used todescribe the nature of the thin semiconductor region underneath theplasmonic waveguide. If the thickness is thin enough to have a quantizeddensity of states (<˜20 nm), only a plasmonic mode would be able to beconfined to such a structure.

One might also consider a hybrid case of a plasmonic-photonic mode,where energy oscillates between photons and kinetic energy in electrons.In this case, the plasmonic waveguide serves in anchoring the opticalmode to the small region of the metal interface, whereas the emissionwill be in a radiative photonic mode. Which type of mode will dominatein the proposed devices depends on the dimensions. For relatively thingain regions (˜20-100 nm), a conventional photonic mode will not besustained, but will leak out, whereas a hybrid photonic-plasmonic modewill likely be the most favorable. For extremely scaled gain regions(˜2-20 nm) only a pure plasmon mode will likely be possible.

The term quantum well may also be designated to refer to one or moreheterojunctions grown extending orthogonally to the plasmonic waveguideand sandwiched in between the n- and the p-region, respectively. Ifthese quantum wells are effectively quantized in two directions thenthey would, for a quantum wire, lead to further quantization of theenergy states.

The term ‘plasmon’ may denote, in physics, a quantum of plasmaoscillation. Just as light (an optical oscillation) consists of photons,the plasma oscillation consists of plasmons. The plasmon may beconsidered as a quasiparticle since it arises from the quantization ofplasma oscillations, just like phonons are quantizations of mechanicalvibrations. Thus, plasmons are collective (a discrete number)oscillations of the free electron gas density. For example, at opticalfrequencies, plasmons may couple with a photon to create anotherquasiparticle, called a plasmon polariton.

Plasmons are bosons which undergo stimulated emission by coupling toelectron-hold pairs (which may be named excitons which are also bosons),e.g., of the gain material. The plasmon quasi-particles that undergo astimulated emission can exist even below the diffraction limit of therelated light (of the same wavelength), i.e., photons. It may also benoted that the dimensions of the active elements (thickness of the gainmaterial or, another relevant quantum well, the plasmonic cavity) laybelow the existence of related photons of the same wavelength/frequency.

The term ‘plasmonic waveguide’ may denote a waveguide for plasmons. Aplasmonic waveguide may denote a cavity that may achieve strong plasmonconfinement. It may be formed by separating a medium of high refractiveindex (usually silicon) from a metal surface (usually gold or silver) bya small physical gap. The plasmonic waveguide or plasmonic cavity mayenable a high-quality, low loss guidance of excited plasmons.

The term ‘dielectric material having a lower refractive index’ maydenote SiO₂, Al₂O₃ or another dielectric below the p-n junction gainmaterial. Such dielectric template may be used to define the quantumwell dimensions and may serve as optical and electrical isolator to anunderlying silicon surface of a used Si substrate.

The term ‘dielectric function’ may be defined in the context ofelectromagnetic permittivity or absolute permittivity. It may denote themeasure of resistance that is encountered when forming an electric fieldin a particular medium. More specifically, permittivity describes theamount of charge needed to generate one unit of electric flux in aparticular medium. Accordingly, a charge will yield more electric fluxin a medium with low permittivity than in a medium with highpermittivity. Thus, permittivity is the measure of a material's abilityto resist an electric field, not its ability to ‘permit’ it (as the name‘permittivity’ might seem to suggest). As opposed to the response of avacuum, the response of normal materials to external fields generallydepends on the frequency of the field. This frequency dependencereflects the fact that a material's polarization does not respondinstantaneously to an applied field. The response must always be causal(arising after the applied field) which can be represented by a phasedifference. For this reason, permittivity is often treated as a complexfunction of the (angular) frequency ω of the applied field. Thus, thedielectric function may have a real and a complex component.

The term ‘nano-wire or nano-tube’ may denote a thin wire having adiameter of 2 nm to 150 nm, more specifically 10 nm to 100 nm and evenmore specifically 30 nm to 50 nm. The nano-wire cross-section may take aplurality of forms, like rectangular, squared, round, elliptic,triangle-shaped, etc.

The term ‘slot-waveguide’ may denote a plasmon waveguide that may guidestrongly confined plasmons in a sub-wavelength-scale. The slot-waveguidemay consist of two strips or slabs of metal separated by asub-wavelength-scale gap region and surrounded by low-refractive-indexcladding materials. The active gain region is located underneath the gapregion of the slot waveguide to maximize the overlap with the stronglylocalized plasmonic mode in this region. The thin dielectric regionunderneath the quantum well gain material further enhances theconfinement to the active region and leads to a reduction of propagationlosses.

The term ‘template assisted selective epitaxy process’ (TASE) may denotea selective epitaxy process useful for compound semiconductordeposition. In TASE, a hollow structure with openings and containing awell-defined nucleation area is fabricated on a wafer surface. Duringthe epitaxy step, the hollow structure (e.g. template) is filled fromthe nucleation area up to the openings, replicating the shape of thetemplate with the semiconductor material.

The proposed semiconductor quantum well laser may offer multipleadvantages and technical effects:

The proposed plasmonic quantum well laser allows for the confinement ofthe propagating mode to a very compact region (below the related photonwavelength) which can be designed to overlap with a narrow electricallypumped region, Thereby, assuring a perfect overlap with the optical modeand the region providing gain. Thus, the problem that the size of the(electrical) pump to the gain material typically exceeds the plasmonicmode area, which reduces the lasing efficiency, is successfullyaddressed. Therefore, an electrically pumped plasmonic nano-laserbecomes a reality that may combine the high attainable gain ofsemiconductor quantum well structures with simulated low-loss plasmonmode to provide highly efficient lasing below the diffraction limit oflight. An optimized electrical injection may be enabled by the growth oflateral p-n junctions in quantum wells which may constitute a novelfeature of the recently developed template assisted selective epitaxy(TASE). Additionally, the proposed structures are well designed for amonolithic integration on silicon.

A further advantage of the very tight modal confinement may be in thefact that one may place the contact regions closer to the active gainregions without causing excessive losses. This may reduce the overallload capacitance which has been a main contribution to the energyconsumption of nanoscale lasers. As a consequence of a reduction of theenergy consumption, required power lines may be reduced in its crosssection area which may be instrumental for increasing the integrationdensity as well as a reduction in produced heat.

It may also be mentioned that the plasmonic waveguide or plasmoniccavity may enable a mode confinement of the plasmons below thediffraction limit and the plasmonic feedback for lasing operations.Furthermore, the dielectric template comprising SiO₂ below the lasingp-n junction may be used for defining the quantum well dimensions andmay serve as optical and electrical isolator to the silicon surfaceunderneath. The quantum well structure itself may provide stronglyenhanced gain due to the density of states modification in the region ofthe p-n junction.

In other words: The plasmonic components may allow for confining theplasmonic mode to much smaller dimensions on the order of 10s of nm, asthe metal anchors the plasmonic mode. Thus, this enables photonicsub-wavelength—i.e., plasmonic components. The present concept thereforeexploits the strong confinement afforded by the plasmonic mode to assurean optimum overlap with the semiconductor region providing gain byelectrical stimulation. The device gets smaller.

In the following, additional embodiments of the plasmonic quantum welllaser will be described:

According to one advantageous embodiment of the plasmonic quantum welllaser, the p-n junction may extend along an entire length of theplasmonic waveguide. This may allow using the so generated plasmons forinformation and data communication purposes on highly integratedelectronic circuits.

According to one embodiment of the plasmonic quantum well laser, anadditional quantum well is positioned between a p-region and an n-regionof the p-n junction. This way, the electrically actuated generation mayhappen in a clearly defined small area which may allow a goodconfinement with the plasmonic waveguide.

According to one permissive embodiment of the plasmonic quantum welllaser, the dielectric material may comprise SiO₂ positioned over asilicon substrate. Thus, the prerequisite that the gain material—i.e.,the p-n junction—is located on a material with lower refractive indexmay be fulfilled. Silicon substrates are standard substrates for VLSIcircuits anyway and the Si wafer or substrate may easily be coated withSiO₂ so to form the basis for the here proposed semiconductor quantumwell laser.

According to one additional embodiment of the plasmonic quantum welllaser, the plasmonic waveguide may be positioned above or atop the p-njunction. A thin dielectric material, like a thin Al₂O₃, HfO₂ or SiO₂layer, may separate the p-n junction from the plasmonic waveguide whichmay typically comprise a metal in order to avoid a shortcut over ther-region and the n-region. However, the thin dielectric material may bebuilt using different approaches: the plasmonic waveguide may be coveredwith the dielectric or the thin dielectric material may cover the p-njunction. The thickness of the thin dielectric material may be in therange of 1 to 5 nm and more specifically in the range of 2 to 3 nm. Sucha thin layer may enable a good plasmoninc confinement to the plasmonicwaveguide, but with manageable losses.

According to one alternative embodiment of the plasmonic quantum welllaser, the plasmonic waveguide may be positioned above or atop—i.e.,having electrical contact—one of the regions of the p-n junction. Thisway, it may be possible to use the plasmonic waveguide as an electricalcontact to the region of the p-n junction it may be in contact with.This may avoid more complex electrical contacts.

According to one possible embodiment of the plasmonic quantum welllaser, the plasmonic waveguide may be a nano-wire. The nano-wire mayhave a rectangular form wire, a triangle or a trapezoid form. This mayallow a good confinement of the plasmon at edges facing the p-njunction. Alternatively, a nano-tube (or a round nano-wire) may be usedas plasmonic waveguide. The thickness of the waveguide may be in therange of 5 to 150 nm and, more specifically in the range of 10 to 100nm, and even more specific in the range of 30 to 50 nm.

According to one permissive embodiment of the plasmonic quantum welllaser, the plasmonic waveguide may be a thin 2-D metal layer. Thethickness of the thin quasi-2-D metal layer may be in a range of 1-20nm. This may allow a confinement of the plasmon along the p-n junctionbelow the metal layer. An embodiment of this kind may easily befabricated because no additional, more complex structures of a 3Dwaveguide or a nano-wire may be required.

According to different embodiments of the semiconductor quantum welllaser, the p-n junction structure may be one selected out of a groupcomprising a lateral p-n junction structure, a lateral p-n-p structure,a lateral p-i-n-i-p structure, a lateral p-NBG-n structure, and alateral p-LBG-n-LBG-p structure, wherein p represents a p-dopedsemiconductor, n represents a n-doped semiconductor, i represents anintrinsic semiconductor, NBG represents a semiconductor with a narrowband gap. It may also be mentioned that the sequence of the p- andn-region may be exchangeable.

Exemplary, useful semiconductor for building the p-n junction maycomprise III-V semiconductors, like InGaAs, GaAs, GaInP, AlGaAs, andsimilar or, III-N junctions, e.g., GaN, as well as II-VI semiconductorjunction. It may be clear to a skilled person that the symbols II, III,V and VI may denote one of the main groups of chemical elements.

According to a further embodiment of the semiconductor quantum welllaser, all junctions—in particular, all p-n junctions of the structuresmay be oriented orthogonally to a longitudinal extension of theplasmonic waveguide. This may allow a good confinement to the structureof the plasmonic waveguide. It may also allow a narrow quantum well(s)between the p- and the n-region, which may in turn result in comparablylow pump power requirements.

According to one useful embodiment of the semiconductor quantum welllaser, the plasmonic waveguide material may comprise plasmonicmetal—e.g. gold, silver or TiN or a highly doped semiconductor—such thatthe dielectric function of the plasmonic waveguide material has anegative real portion of a related dielectric function. Again, this mayallow the propagation of a surface plasmon polariton (SPP) mode. As areminder: a plasmonic metal may be a material having a negative portionof its dielectric function.

According to one practical embodiment of the semiconductor quantum welllaser, the plasmonic waveguide may be a slot-waveguide, wherein oneportion (one rail) of the slot-waveguide may be positioned above oratop-atop meaning in direct electrical contact—an n-region of the p-njunction and another portion (the other rail) of the slot-waveguide maybe positioned above or atop a p-region of the p-n junction, wherein theslot-waveguide confining an electromagnetic field to the p-n junction.With the portion being positioned directly in electrical contact withthe regions of the p-n junction, the individual portion may be directlyused as electrical contacts to the regions itself. No additionalelectrical contacts to the regions of the junction structure may berequired. This may save costly space in the LSI circuit.

According to one advantageous embodiment of the semiconductor quantumwell laser, the semiconductor quantum well laser is a result of atemplate assisted selective epitaxy (TASE) process. This process may beinstrumental to fabricate the required thin semiconductor layer for thep-n junction(s). The so laterally grown p-n junction represents a uniquefeature of the TASE process which enables a strong overlap of theplasmon mode with the p-n recombination zone.

A growth seed used to fabricate the laterally grown p- and n-regions ofthe p-n junction may enable a monolithic integration of optically activesemiconductor material on silicon as electrical contacts.—According toan alternative advantageous embodiment of the plasmonic quantum welllaser, the semiconductor quantum well may be fabricated by epitaxy andlayer transfer.

In the following, a detailed description of the figures will be given.All instructions in the figures are schematic. Firstly, a block diagramof an embodiment of the inventive semiconductor quantum well laser isgiven. Afterwards, further embodiments, as well as embodiments of themethod for building a semiconductor quantum well laser, will bedescribed.

FIG. 1 shows a block diagram of an (sort of abstract) embodiment of theinventive semiconductor quantum well laser including a plasmonicwaveguide. A semiconductor quantum well laser 100 including a plasmonicwaveguide 108 comprises a p-n junction structure 104, 106 extendingorthogonally to a direction of plasmon propagation along the plasmonicwaveguide 108—i.e., from the font to the back of the 3D sketch of theshown structure (or back). Thereby, the p-n junction 104, 106 ispositioned atop a dielectric material 102 having a lower refractiveindex—e.g., SiO₂—than material building the p-n junction (like thementioned III-V, and I-VI semiconductor material examples mentionedabove). The quantum well laser is electrically pumpable via electricalcontact to the p-region 104 and the n-region 106 of the p-n junctionstructure. It may be noted that in the recombination area 110 noelectrical contact of the plasmonic waveguide 108 to the p-region 104and/or the n-region 106 is enabled in order to avoid an electricalshortcut between then p-region 104 and the n-region 106.

FIG. 2 shows a general schematic illustration 200 of a plasmonicwaveguide 108 on top of a horizontally oriented quantum well layer inwhich the quantum well is typically oriented vertically to the plasmonicwaveguide 108. The quantum well 204 layer may, e.g., be implemented as aIII-V gain material over a low index dielectric material 202 (e.g., inform of SiO₂). On top of the quantum well, the plasmonic waveguide 108is shown schematically as a rectangle. The plasmonic waveguide 108 maybe surrounded on three sides (not the lower side facing the quantumwell) by natural air or vacuum. In such a setup, simulations show thatplasmons 208 can be strongly confined to the very thin (few nm) layer ofgain material 204 if the gain material 204 is sandwiched between a lowindex dielectric 202 and the proximal plasmonic waveguide 108. It can beshown that the plasmonic mode 208 is strongly confined to the loweredges of the plasmonic waveguide 108 within the quantum well gainmaterial. It is also assumed that the quantum well has a comparably highrefractive index—in particular, if compared to the comparable low indexof the underlying SiO₂. It may be clear to a skilled person that theplasmonic waveguide may not be in direct contact with both sides of ap-n junction in the III-V gain material (because it may cause ashortcut). Details are shown in the following, more realisticembodiments. The coordinate system, integrated into FIG. 2, may give anindication of the dimensions of the plasmonic waveguide 108 and of aquantum well in the gain material 204.

FIG. 3 shows an embodiment 300 with a metallic nano-wire waveguide 108on a simple p-n junction 104, 106. The metallic nano-wire waveguide 18may be surrounded by or coated with a thin (few nm, e.g., 1 to 5 nm)dielectric material 302—e.g., SiO₂—separating the metal of the metallicnano-wire waveguide 108 and the p-n junction 104, 106 to avoid anelectrical shortcut over the p-n junction. A voltage V_(bias) and aground connection are also shown. The p-n junction is formed by ap-region 104 and an n-region 106. Also shown are a pump voltage V_(bias)connection 304 to the p-region 104, as well as a ground connection 306to the n-regions 106.

FIG. 4 shows an alternative embodiment 400 in which the metallicnano-wire waveguide 108 is only positioned on one side of the p-njunction, here the n-region 106. In this case, it is not necessary toseparate the metallic nano-wire of the metallic nano-wire waveguide 108from the semiconductor quantum well layer composed of the p-n junctionlayer 104, 106. In this case, the metallic waveguide may be used as aground connection 306 while the voltage V_(bias) requires a separateconnection to the p-region 104 of the p-n junction 104, 106 structure.This embodiment is a technologically simple implementation, but is lessadvantageous from a performance perspective as only the plasmon modeclose to the junction has an optimal overlap with the pumped region.

FIG. 5 shows again an alternative embodiment 500 with the metallicnano-wire waveguide being coated with a dielectric 502, e.g., SiO₂. Inthis case, a plurality of quantum wells 504 (or only one) is shownbetween the p-region 104 and the n-regions of the p-n junction asvertical stripes between the p- and the n-region of the junctionstructure.

FIG. 6 shows a further alternative embodiment 600 in which the nano-wirewaveguide 108 is shown on a very thin dielectric material 602 a, and thegain material containing the p-n junction is buried underneath. The gainmaterial—i.e., the p-region 104 and the n-region 106—may be buried inthe SiO₂ bed 602.

FIG. 7 shows an additional alternative embodiment 700, wherein themetallic waveguide 108 (exemplary shown with a rectangular thecross-section) is a double junction, i.e., a pnp-junction. An advantageof such architecture is that the metallic waveguide 108 may bepositioned directly atop and in electrical contact with the middlen-region. It may be used as ground connection 306. V_(bais) connections304, 708 are connected to the p-regions 104 and the additional p-region702. It may be clear that instead a pnp double junction also an npndouble junction may be used.

FIG. 8 shows a further implementation option 800 in which the metallicwaveguide 108 is positioned atop a pnp double junction with verticallyoriented quantum well regions 802, 804 at the individual p-n junction704 and the n-p junction 706. The figure shows also clearly theadditional p-region 702. Also here, it may be clear that instead a pnpdouble junction also an npn double junction may be used.

FIG. 9 shows another embodiment 900 as a plane metallic layer 902 on adielectric material layer 904. This layer 904 may also be in contactwith the burying material 906 burying the p-n junction 104, 106 whichdefines the gain region of the semiconductor laser. This way, thepropagating plasmons will be concentrated over the recombination area ofthe p-n junction structure 104, 106. In this case the laser cavity isdetermined by the geometrical extension of the buried gain region (=p-njunction 104,106), rather than by patterning the plasmonic waveguide.

FIG. 10 shows a special embodiment 1000 with the metallic slotwaveguide. Here, the plasmon field propagates in the opening between thetwo triangle shaped partial waveguides 1002, 1004, i.e., in the openingbetween the partial waveguides 1002, 1004. Each side of the slot servesadditionally the purpose of functioning as a metallic contact to the p-and the n-region, respectively.

FIG. 11 shows process steps of an embodiment 1100 of a method forbuilding the plasmonic quantum well laser, using the Template-AssistedSelective Epitaxy (TASE). This will be explained shortly: Firstly,referring to FIG. 11(a), an SOI wafer (Si-bulk material 1102 underSiO₂-layer 1104 topped by a Si layer 1106), thinned to the desiredthickness of the gain material. If the later built gain material is thinenough (<˜20 nm) this also acts as a quantum well.

Secondly, referring to FIG. 11(b), a pattern 1108 for defining the shapeof the laser gain material (to be grown later on) is patterned into thetop silicon layer 1106 by conventional lithographic patterningtechniques.

Thirdly, referring to FIG. 11(c), the entire structure is covered withan oxide (not shown, SiO₂, or other dielectric template layer), which isopened on one end 1114 to selectively etch away part of the silicon,thereby creating a hollow template cavity 1110. At one extremity of thecavity a narrow ˜20-100 nm wide silicon seed 1112 is exposed.

In a next step, referring to FIG. 11(d), one side or portion 1116 of thep-n junction (e.g., a p-region) is epitaxially grown extending from thesilicon seed 1112. In a further step, referring to FIG. 11(e), the otherpolarity side or portion 1118 (e.g., an n-region) of the p-n junction1120 is grown, thereby completing the gain region of the plasmonicquantum well laser. The cavity may be completely filled or it may befilled until the desired expansion is obtained. The growth of thep-junction is likely done in one step with a variation of precursorgasses. In-between the two regions 1116, 1118 of the p-n junction 1120one may incorporate—an un-doped i-region, a heterojunction, or a secondquantum well according to the embodiments (compare, e.g., FIG. 5, FIG.8). Last but not least, referring to FIG. 11(f), the template oxide isstripped away and the plasmonic waveguide 108 is patterned as describedin more detail in the context of the previous figures. Electricalcontact can be provided to the regions of the p-n junction and/or theplasmonic waveguide 108 as described in the context of the previousfigures.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinaryskills in the art without departing from the scope and spirit of thedescribed embodiments. The terminology used herein was chosen to bestexplain the principles of the embodiments, the practical application ortechnical improvement over technologies found in the marketplace, or toenable others of ordinary skills in the art to understand theembodiments disclosed herein.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

The flowcharts and/or block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or act or carry out combinations of special purpose hardwareand computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the invention. As usedherein, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will further be understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or steps plus function elements in the claims below are intendedto include any structure, material, or act for performing the functionin combination with other claimed elements, as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skills in the artwithout departing from the scope and spirit of the invention. Theembodiments are chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skills in the art to understand the invention forvarious embodiments with various modifications, as are suited to theparticular use contemplated.

What is claimed is:
 1. A plasmonic quantum well laser comprising aplasmonic waveguide, said plasmonic quantum well laser comprising a p-njunction structure extending orthogonally to a direction of plasmonpropagation along said plasmonic waveguide, wherein said p-n junction ispositioned atop a dielectric material having a lower refractive indexthan material building said p-n junction, and wherein said plasmonicquantum well laser is electrically actuated.
 2. The plasmonic quantumwell laser according to claim 1, wherein said p-n junction extends alongan entire length of said plasmonic waveguide.
 3. The plasmonic quantumwell laser according to claim 1, wherein an additional quantum well ispositioned between a p-region and an n-region of said p-n junction. 4.The plasmonic quantum well laser according to claim 1, wherein saiddielectric material comprises SiO₂ positioned over a silicon substrate.5. The plasmonic quantum well laser according to claim 1, wherein saidplasmonic waveguide is positioned above or atop said p-n junction. 6.The plasmonic quantum well laser according to claim 1, wherein saidplasmonic waveguide is positioned above or atop one of said regions ofsaid p-n junction.
 7. The plasmonic quantum well laser according toclaim 1, wherein said p-n junction and said plasmonic waveguide areseparated by a thin dielectric material.
 8. The plasmonic quantum welllaser according to claim 1, wherein said plasmonic waveguide is anano-wire or a nano-tube.
 9. The plasmonic quantum well laser accordingto claim 1, wherein said plasmonic waveguide is a thin 2-D metal layer.10. The plasmonic quantum well laser according to claim 1, wherein saidp-n junction structure is one selected out of a group comprising alateral p-n junction structure, a lateral p-n-p structure, a lateralp-i-n-i-p structure, a lateral p-NBG-n structure, and a lateralp-NBG-n-NBG-p structure, wherein p represents a p-doped semiconductor, nrepresents a n-doped semiconductor i represents an intrinsicsemiconductor NBG represents a semiconductor with a narrow band gap. 11.The plasmonic quantum well laser according to claim 10, wherein alljunctions of said structures are oriented orthogonally to a longitudinalextension of said plasmonic waveguide.
 12. The plasmonic quantum welllaser according to claim 1, wherein said plasmonic waveguide materialcomprises metal, wherein said dielectric function of said plasmonicwaveguide material has a negative real portion of a related dielectricfunction.
 13. The plasmonic quantum well laser according to claim 1,wherein said plasmonic waveguide is a slot-waveguide, wherein oneportion of said slot-waveguide is positioned above or atop an n-regionof said p-n junction and another portion of said slot-waveguide ispositioned above or atop a p-region of said p-n junction, saidslot-waveguide confining an electromagnetic field to said p-n junction.14. The plasmonic quantum well laser according to claim 1, wherein saidsemiconductor quantum well laser is a result of a template assistedselective epitaxy process.
 15. A method for building a plasmonic quantumwell laser, said method comprising providing an SiO₂ layer over an Sibulk material, wherein an Si layer is deposited atop of said SiO₂ layer;patterning an Si structure allotted for a gain region into said Silayer, such that said Si structure forms a pattern atop said SiO₂ layer;covering said Si structure with an SiO₂ template layer; building anopening in said SiO₂ template layer from one side and etching away saidSi pattern, saidreby building a cavity within said SiO₂ template layer;epitaxially growing horizontally a first semiconductor portion of a p-njunction extending from a silicon seed exposed within said SiO₂ templatetowards said opening until a predefined horizontal extension is reached;growing horizontally a second portion of said p-n junction through saidopening extending from an end of said predefined horizontal extensionfacing said opening until said cavity is filled, such that said p-njunction builds said gain region of said plasmonic quantum well laser;depositing a dielectric material over said p-n junction; and depositinga plasmonic waveguide over said dielectric material along a longitudinalextension of said p-n junction.